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| United States Patent |
5,008,337
|
|
Patel
|
April 16, 1991
|
Method of crosslinking rubber
Abstract
Crosslinking of rubber having a carboxylic functionality is accomplished by
heating the rubber in the presence of an isocyanate functionalized
compound and an accelerator which is a metal salt of an organic acid. The
crosslinked polymers are useful in a variety of shaped articles, and can
be in the form of dispersed particles, in a continuous plastic phase.
| Inventors:
|
Patel; Raman (Akron, OH)
|
| Assignee:
|
Monsanto Company (St. Louis, MO)
|
| Appl. No.:
|
424413 |
| Filed:
|
October 20, 1989 |
| Current U.S. Class: |
525/130; 525/123; 525/126; 525/127 |
| Intern'l Class: |
C08L 013/00; C08L 025/02 |
| Field of Search: |
525/130,123,127,126
|
References Cited
U.S. Patent Documents
| 3179716 | Apr., 1985 | Bruin | 525/130.
|
| 4046745 | Sep., 1977 | Selman et al. | 260/77.
|
| 4202950 | May., 1980 | Statton | 525/123.
|
| 4574140 | Mar., 1986 | Sandstrom et al. | 525/123.
|
| 4742113 | May., 1988 | Gismondi et al. | 524/762.
|
Primary Examiner: Short; Patricia
Attorney, Agent or Firm: Seward; Gordon B.
Parent Case Text
This application is a continuation in part of copending application Ser.
No. 179,814, filed Apr. 11, 1988, now abandoned.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. The method of crosslinking a carboxylic-functionalized rubber comprising
heating the rubber in the presence of a polyfunctional isocyanate
functionalized compound and an accelerator which is a metal salt of an
organic acid.
2. The method of claim 1 wherein the rubber is a copolymer from a C.sub.1
-C.sub.4 alkyl acrylate and an alpha olefin of 2-8 carbon atoms.
3. The method of claim 2 wherein the rubber is a copolymer from ethylene
and methyl acrylate.
4. The method of claim 3 wherein the carboxylic functionality is provided
by a termonomer which is an unsaturated carboxylic acid or a half-ester of
an unsaturated dicarboxylic acid.
5. The method of claim 4 wherein the termonomer is monoethyl maleate.
6. The method of claim 5 wherein the copolymer rubber comprises at least
about 30 mole percent ethylene, about 10 to 69.5 mole percent methyl
acrylate and about 0.5 to 10 mole percent monomethyl maleate.
7. The method of claim 1 wherein the isocyanate functionalized compound is
toluene diisocyanate or an isocyanate-terminated polyester prepolymer.
8. The method of claim 1 wherein the accelerator is a metal salt of stearic
acid or acetic acid.
9. The method of claim 8 wherein the accelerator is magnesium stearate.
10. The method of claim 9 wherein the rubber is a copolymer from ethylene,
methyl acrylate and monomethyl maleate and the isocyanate functionalized
compound is an isocyanate-terminated polyester prepolymer.
Description
The invention relates to a method for crosslinking rubber which is
acid-functional, preferably an acid-functional copolymer from alpha olefin
monomer and acrylate monomer. Crosslinking is accomplished with an
isocyanate functionalized compound, accelerated by a metal salt of an
organic acid.
The acid-functional rubbers which are crosslinked by the method of the
invention include ethylene-acrylic elastomers. The crosslinking of these
rubbers has been accomplished with primary diamines, or with peroxides,
although the diamines are preferred. Isocyanate-functionalized compounds
or polymers have also been used for this purpose.
BRIEF SUMMARY OF THE INVENTION
It has now been discovered that acid-functional rubbers, especially
acid-functional copolymer rubbers from alpha olefin monomer and acrylate
monomer, can be much more rapidly cross-linked with isocyanates by
accelerating the isocyanate-crosslinking with a metal salt of an organic
acid.
DETAILED DESCRIPTION
The acid-functional copolymer rubbers used in the method of the invention
are acid-functionalized diene homopolymers or copolymers with styrene or
acrylonitrile, or EPDM rubber. Preferred rubbers are copolymers from alpha
olefin monomer and alkyl acrylate monomer. Suitable alpha olefins for
polymerization of such copolymer rubbers include ethylene, propylene,
butene-1, isobutylene, pentenes, heptenes, octenes, and the like or
mixtures thereof; C.sub.1 -C.sub.4 alpha olefins are preferred and
ethylene is often most preferred. Suitable alkyl acrylates for
copolymerizing with the alkene include methyl acrylate, ethyl acrylate,
t-butyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, and the like or
a mixture thereof; C.sub.1 -C.sub.12 alkyl acrylates are often preferred,
and C.sub.1 -C.sub.4 alkyl acrylates are most often preferred. In many
cases a preferred olefin/acrylic ester copolymer rubber comprises
unsaturated carboxylic acid monomer units, such as acid units, e.g.
derived from (meth)acrylic acid or maleic acid, anhydride units, e.g.
derived from maleic anhydride or partial ester units, e.g. derived from
mono ethyl maleate. In many cases a preferred olefin/acrylic ester
copolymer rubber is a terpolymer of ethylene, C.sub.1 -C.sub.4 alkyl
acrylate and an acidic monomer unit; more preferably such terpolymer
comprises at least about 30 mole percent of ethylene, about 10 to 69.5
mole percent of methyl acrylate and about 0.5 to 10 mole percent mono
ethyl maleate. Suitable rubbers are described on pages 325-334, Volume 1
of the Encyclopedia of Polymer Science and Engineering (2nd Ed).
Commercial rubbers of this type are sold by du Pont under the trademark,
VAMAC. In all cases it is preferred that the rubber be essentially
non-crystalline and have a glass transition temperature (T.sub.g) below
room temperature, i.e. below about 23.degree.. Other, less preferred
rubbers include carboxylicfunctional nitrile rubber, polyisoprene,
polybutadiene, styrene-butadiene rubber, EPDM rubber or hydrogenated
derivatives of any of these rubbers.
Crosslinking agents employed in the method of this invention are
polyfunctional, i.e. at least difunctional, compounds selected to cure the
rubber, i.e. crosslink the rubber, by covalently bonding with the reactive
functional groups of the rubber. The covalent crosslinking agent is a
compound with an isocyanate reactive functional group. Effective
crosslinking agents include isocyanates such as toluene di-isocyanate or
isocyanate-terminated polyester prepolymers. Generally, the amount of
crosslinking agent does not exceed about 15 percent by weight of the
copolymer rubber, depending on the molecular weight of the rubber and
crosslinking agent. Preferred amounts of crosslinking agent are readily
determined by routine experimentation to optimize desired properties of
the compositions of this invention. The amount of crosslinking agent and
the degree of crosslinking can be characterized in terms of the cure rate
parameters as measured by a curemeter or rheometer, as is well known in
the rubber industry.
The accelerators used in the method of the invention are metal salts of
organic acids. Preferred organic acids are carboxylic acids of from 2 to
20 carbon atoms, including as formic, acetic, propanoic, butanoic, lauric,
oleic, stearic, palmitic and undecanoic acids. Most preferred is stearic
acid. The metals which form the preferred salts include calcium, copper,
aluminum, cadmium, cobalt and magnesium, with magnesium being especially
preferred.
The method comprises heating the rubber in the presence of the crosslinking
agent and the accelerator. As indicated above the amounts used are readily
determined by experimentation, but are usually in the range of from 0.1 to
25 parts of crosslinking agent and 0.1 to 5 parts of accelerator by weight
per 100 parts of rubber by weight. The time and temperature of such
treatment can also be easily determined. Preferred temperatures range from
above room temperature to below the decomposition temperature for the
rubber, and are usually in the range of from 100.degree. to 250.degree. C.
Higher temperatures permit shorter treatment times; typically the highest
practical temperature will be used, in order to minimize the time
required.
The ingredients, rubber, crosslinking agent and accelerator should be
intimately mixed, using standard rubber mixing equipment, such as mills
and internal mixers. Care should be taken during mixing not to prematurely
activate the crosslinking step, which is typically accomplished after the
rubber is shaped into a finished product, as in a mold, or by extrusion or
calendering. In an alternative method, the rubber can be dispersed in a
continuous plastic phase, such as polyolefin or polyester, and vulcanized
under shear to produce a thermoplastic elastomer.
A better understanding of the invention will be obtained by reference to
the following examples in which all parts are by weight and all
temperatures are in degrees celsius, unless otherwise indicated.
EXAMPLE 1
In order to evaluate the method of the invention, a number of rubber
compounds were prepared and tested. Preparation was done by admixing the
ingredients in a laboratory mill at room temperature, with a number of
different crosslinking agents and accelerators. After mixing, samples of
the compounds were tested at 180.degree. C. in an Oscillating Die
Rheometer, to determine their cure parameters. Several crosslinking agents
and accelerators outside the scope of the method of the invention were
also incorporated and tested, as comparisons. The formulations and test
results are set forth in Table I. In all cases, the rubber was VAMAC-G,
believed to be a neat copolymer from ethylene, methyl acrylate and
monoethyl maleate.
The following abbreviations were used for the materials:
______________________________________
Material Abbreviation
______________________________________
Acetate (salt) A
Stearate (salt) ST
Methylene dianiline
MDA
Toluene diisocyanate
TDI
Diphenyl guanidine
DPG
Dicumyl peroxide DICUP
______________________________________
Mondur E-501 (MON) is an isocyanate-terminated polyester prepolymer
containing about 19% NCO. Aclyn 246 (ACL) is a low molecular weight
magnesium ionomer, manufactured by Allied Signal, based on an ethylene
polymer. HVA is metaphenylene bis-maleimide.
The rheometer results are expressed in terms of time and torque parameters,
which characterize the curing behavior of the rubber in the presence of
the different crosslinkers and accelerators, and their levels. The
following terms are used:
ML-minimum torque, in pound-inches.
MH-maximum torque, in pound-inches.
TS.sub.2 -time, in minutes and seconds, to reach two chart divisions above
minimum torque.
T.sub.50 -time, in minutes and seconds, to reach 50% of maximum torque.
T.sub.90 -time in minutes and seconds, to reach 90% of maximum torque.
TABLE I
______________________________________
Curative
Phr Activator
Phr t50 t90 ML MH
______________________________________
MON 10 None -- 6:29 20:06
0.08 0.71
MON 10 CuAc .25 5:42 15:34
0.07 1.45
MON 10 CoAc .25 9:11 21:18
0.07 2.33
MON 10 MgSt .25 3.17 8.35
0.10 7.97
MON 10 CaSt .25 5:18 10:59
0.07 5.00
MON 10 CoSt .25 9.41 23:39
0.07 2.09
MON 10 AlSt .25 13:35 23.31
0.06 3.01
MON 10 CdSt .25 7.44 17.02
0.04 4.22
MON 10 ACL 2 11:06 24:25
0.06 1.02
MDA 1.25 DPG 4 4:59 11:02
0.05 3.54
DICUP 7 HVA 2 1:05 3:02
0.10 2.83
MON 7.5 MgSt .25 2:28 4:58
0.08 2.86
TDI 4 MgSt .25 2:45 6:43
0.08 3.55
______________________________________
The data in Table I show that, based on both time and torque, Mondur E501
by itself produces only a modest increase in torque, over a long span of
time. The addition of a small amount of any of the activators of the
invention results in significantly higher maximum torque values, while the
minimum torque values are essentially unaffected. The control
compositions, 10 and 11, show that, at equivalent maximum torque, the
combination of dicumyl peroxide and m-phenylenebismaleimide gives
extremely fast cures, which are different to handle. A control experiment
(not shown) using magnesium chloride gave no increase in maximum torque
over the control with Mondur E501 and no activator.
Although the invention has been illustrated by typical example, it is not
limited thereto. Changes and modifications of the examples of the
invention herein chosen for purposes of disclosure can be made which do
not constitute departure from the spirit and scope of the invention.
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